THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

[18:00:02]

WOLF BLITZER, CNN ANCHOR: How will the Supreme battle play out?

The front-runner, Donald Trump, calling on Republicans to unite behind him after his latest string of primary victories and predicting riots, riots, if he's blocked at the party convention. Will he bring the GOP together or fracture the party beyond repair?

The spoiler, John Kasich now saying he's the only Republican candidate who can win the general election after reviving his struggling campaign with a desperately needed primary victory. But he's far behind Trump and Ted Cruz in the delegate count. Can the only establishment candidate left in the race pose a credible challenge?

An attack thwarted? New terror arrests in Paris and a deadly terror raid in Brussels, police recovering weapons, ammunition and an ISIS flag. One suspect killed in a gun battle with police, but two others escape and are on the loose. Tonight, are they plotting a new terrorist strike?

We want to welcome our viewers in the United States and around the world. I'm Wolf Blitzer. You're in THE SITUATION ROOM.

ANNOUNCER: This is CNN breaking news.

BLITZER: We're following the breaking political news, including the sudden cancellation of the next Republican presidential debate, as front-runner Donald Trump says he will not participate.

His momentum stronger than ever tonight after dominating the latest round of primaries, and Trump now saying riots are possible if he's denied the nomination.

We're also following the battle over the U.S. Supreme Court vacancy. Senate Majority Leader Mitch McConnell says he's spoken by phone to President Obama's nominee, Merrick Garland, chief judge for the U.S. Court of Appeals here in Washington. McConnell says he told Garland the Senate will not take action on his nomination.

And there's breaking terror news tonight. Police in Paris arresting four people suspected of plotting an attack. Three men and one woman, they are in custody one day after a deadly terror raid in Brussels. One terror suspect was killed in that operation. Two others escaped.

We're covering all of that, much more this hour with our guests, including Republican Congressman Jason Chaffetz. Our correspondents and expert analysts, they are also standing by.

Let's begin with the Republican race for the White House and a new political landscape tonight after the latest primaries.

CNN's Phil Mattingly has all the late-breaking developments for us.

Phil, it's down to three Republican candidates tonight.

PHIL MATTINGLY, CNN CORRESPONDENT: Three Republican candidates, Wolf, all three believe they are the man that the party should rally behind, but only one front-runner. And that front-runner is setting the terms both for debates and in a warning for what could happen next if Republican leaders push a contested convention.

(BEGIN VIDEOTAPE)

MATTINGLY (voice-over): Tonight, Donald Trump is calling for the Republican Party to coalesce behind him, even as his path to the nomination remains complicated.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: The fact is, we have to bring our party together. We have to bring it together. We have something happening that actually makes the Republican Party probably the biggest political story anywhere in the world.

MATTINGLY: The GOP front-runner issuing a stark warning if he keeps his sizable delegate lead and Republican leaders turn to a different nominee.

TRUMP: If we're 100 short, and we're at 1,100 and somebody else is at 500 or 400, because we're way ahead of everybody, I don't think you can say that we don't get it automatically. I think you would have riots.

MATTINGLY: Just days after his rivals criticized him for encouraging violence at his rallies. After another huge Super Tuesday showing, notching wins in three states, Trump pledged today to skip the next scheduled GOP debate Monday night.

TRUMP: How many times can the same people ask you the same question? So I was very surprised when I heard that FOX called for a debate. Nobody told me about it. And I won't be there, no.

MATTINGLY: With John Kasich balking at a Trumpless event, FOX News eventually pulled the plug. Trump's dominant victory in Florida was the final crushing to Marco Rubio's once promising campaign.

SEN. MARCO RUBIO (R-FL), PRESIDENTIAL CANDIDATE: The fact that I have even come this far is evidence of how special America truly is.

MATTINGLY: The Florida senator leaving the race with a clear message for Republican voters. RUBIO: America needs a vibrant conservative movement, but one that's

built on principles and on ideas, not on fear, not on anger, not on preying on people's frustrations.

(CHEERING AND APPLAUSE)

MATTINGLY: Trump's only setback Tuesday night coming in Ohio, where home state Governor Kasich picked up a convincing win, his first of the campaign.

GOV. JOHN KASICH (R-OH), PRESIDENTIAL CANDIDATE: We are going to go all the way to Cleveland and secure the Republican nomination.

(CHEERING AND APPLAUSE)

MATTINGLY: Kasich hitting the trail today in Pennsylvania.

KASICH: For the first time, people are actually beginning to see my name, my face, and hear my message.

MATTINGLY: Ted Cruz, pulling in more delegates and holding onto a clear second-place position, dismissing Kasich's claim that the GOP fight is still a three-man race.

[18:05:00]

SEN. TED CRUZ (R-TX), PRESIDENTIAL CANDIDATE: Only two campaigns have a plausible path to the nomination, ours and Donald Trump's. Nobody else has any mathematical possibility whatsoever.

MATTINGLY: And pitching a renewed push for party leaders to unite behind his campaign and take out Trump.

CRUZ: We're seeing Republicans unite behind our campaign, because we're the only campaign that is beating Donald Trump over and over and over again, and we're the only campaign that can and will beat Donald Trump.

(END VIDEOTAPE)

MATTINGLY: Wolf, for all the attention and money that has been sent over to the stop Trump effort, so far, lacking in results and really only one mathematical option that a lot of advisers are pointing out to actually get that job done. And that's laying on a contested convention.

Ted Cruz still sticking to his position that he believes he can beat Donald Trump straight up. John Kasich here and just outside of Philadelphia today maintaining that that is the place where he can beat Donald Trump. Either way, Wolf, all we know is one thing is for certain. We have a long slog ahead with these three candidates still in the race.

BLITZER: I suspect you're right. All right, Phil, thank you very much.

Let's get some more on the Cruz campaign right now.

CNN's Sunlen Serfaty is joining us from Houston.

Sunlen, the Cruz campaign still believes -- we just heard -- there's a path forward for them. What are you hearing?

SUNLEN SERFATY, CNN NATIONAL CORRESPONDENT: Well, that's right. They do think that there is a path forward, but, Wolf, Cruz campaign officials do acknowledge that it is going to be a difficult, but doable path forward.

Right now, the Cruz campaign ISIS targeting Utah. They are targeting Wisconsin and also the upcoming states that have closed primaries or caucuses. These are contests that largely have favored Cruz over Trump in the past.

They are still predicting that they can get to that magic number, 1,237, outright and before any sort of contested convention. Of course, that narrow path comes with -- a lot more narrow with John Kasich in the race. The Cruz campaign feels that he splinters the anti-Trump vote.

But, interesting, when Cruz was asked if he's going to go after John Kasich on the trail, he said no. He in his mind largely thinks this is still a two-man race. He's largely going to ignore John Kasich, except to call for his supporters to come to his campaign -- Wolf.

BLITZER: As you know, Sunlen, South Carolina Governor Nikki Haley is indicating she's shifting her support now to Ted Cruz. Is the Cruz campaign confident a majority of other key Rubio supporters, like the governor, will do the same thing?

SERFATY: Well, they hope so. And certainly South Carolina Governor Nikki Haley's statement today a big signal that the Cruz campaign of course would welcome, saying that she hopes that this will -- Cruz will come through in the end, but really stopping short of a formal and outright endorsement of Ted Cruz.

But, certainly, the Cruz campaign wants to woo these Rubio supporters to their side. And that's why we saw Ted Cruz in his speech here in Houston last night say point blank to the Rubio supporters, come to our side. We welcome you with open arms.

According to Cruz campaign internal documents, they believe that 80 percent -- in a head-to-head matchup, 80 percent of voters that were Rubio supporters will come to them over Donald Trump. But, of course, that is contingent on John Kasich not being in the race, which is why he's becoming such a thorn in the side for the Cruz campaign right now, Wolf.

BLITZER: I suspect a lot of those Rubio supporters will go to John Kasich for the time being.

All right, thanks very much, Sunlen, for that.

Let's get some more on all of this. Joining us, Republican Congressman Jason Chaffetz of Utah. He's the chairman of the Oversight and Government Reform Committee. He's a member of the Judiciary Committee. And he was a supporter of Senator Marco Rubio.

Congressman, thanks very much for joining us.

So where do you go from here? Whose campaign are you going to support now?

REP. JASON CHAFFETZ (R), UTAH: Well, look, the voters get to make these decisions.

I'm probably not going to endorse in one way or the other. I'm going to focus on doing what I was elected to do. And at this point, I don't know that endorsements mean a whole lot. I think a lot of voters have already made up their mind. I think they mean something maybe before the first vote is cast.

And I supported Marco Rubio. I still think he was our best bet to beat the Democrats in November, but it's not to be.

BLITZER: As you know, in your home state Utah, the Republican caucuses next Tuesday in Utah. I assume you are going to participate and decide which one of these three Republicans candidates you will support. Right?

CHAFFETZ: Yes, I think, look, Donald Trump, it feels like, is snubbing Utah, isn't going to show up to the debate, isn't coming to our state. That's not going to play well with a state that is usually a flyover state.

But when you have an actual contest, you know, when you have Trump out and then Kasich saying, well, I'm not going to come, that looks pretty good for Ted Cruz in such a tight race. And I think Utahans will look favorably upon that.

CHAFFETZ: Well, yes, like I said, I'm not endorsing. I'm going to stay out of it, but, look, I want to put forward our best foot so that we can actually win the White House in November.

BLITZER: You are a smart guy, but, realistically, do you think either Cruz or Kasich can really stop Trump from getting the Republican nomination?

[18:10:02]

CHAFFETZ: Well, yes.

We have just -- he's gone -- Mr. Trump has gone just past the halfway point. But you actually have to win the race. If the horses are going around the track, you actually have to bring that horse across the finish line. And so this whole idea of riots and all that, that's just kind of silly, childish talk here.

You got to make your best case that you can. You have got to win on policy. And we vote on it. The Republican side of this equation is very different than the Democrats. There's not some master Republican puppeteer who is the establishment making the calls on this.

We don't have superdelegates, like they do on the Democratic side of the equation, where you get all these people that get votes at their convention that weren't elected at their caucus or through the primary systems. So, we're very different here and we're going to see how the voters take it. There's still a lot of action yet to be played out on the field.

BLITZER: He is the front-runner right now, Donald Trump. Let's say he is the Republican nominee. What do you think happens to the Republican Party?

CHAFFETZ: Well, look, I intend to support the nominee.

And when ultimately you get down to the choice of, say, Hillary Clinton or Donald Trump, the question is going to be, do you want Hillary Clinton to be the president of the United States? I think the majority of Americans say, heck, no, we don't trust her. She's not a trustworthy person. We want to go a different direction, somebody who is going to do the right things for the United States of America.

So I intend to support the Republican nominee, but I'm going to stay out of that fight until we figure out who that is going to be.

BLITZER: Have you had a chance to talk to Senator Rubio since his decision last night to drop out of the race? What's your sense of his political future?

CHAFFETZ: No, look, he's got a bright future. He's a young man. He's 44 years old and has accomplished amazing things.

I'm very proud of him and what he's been able to accomplish and keep his family intact, I think, keep his integrity in place. Certainly, that speech last night made us all proud. And I think it showed the country why there were so many of us by literally the millions that support Marco. And he's got an important voice for the Republican Party moving forward.

BLITZER: Why do you think Donald Trump has done so well in all these Republican primaries and caucuses?

CHAFFETZ: Look, he's an attitudinal candidate is the way I refer to it.

He's expressing the attitude that I think the majority of America feels like. They are frustrated. They're mad. They want to see results. They want America to stand strong in the world. They want somebody to call it out the way that they see it. He's got a lot to offer, and he is bringing in a group of people that have not participated in this process in the past and maybe haven't voted for Republicans in the past. And I think he makes a very valid point when he says we should be

thanked for bringing a lot of new people. And we're going to need that in November. And he's got a very strong case to make in that regard.

BLITZER: And when he says, as he said this morning on CNN, on "NEW DAY" with Chris Cuomo, that let's say he doesn't get 1,237. That's the number you need to automatically get the Republican presidential nomination at the convention in Cleveland, but he gets close.

Let's say he's 100 shy. Do you think it's realistic at all that you can deny him the nomination?

CHAFFETZ: Well, again, that's why you have these contests, and people are going to have to flip one direction or another.

It's hard for me to predict, where is that threshold? How close is -- let's say hypothetically, how close is Ted Cruz? If Donald Trump is 100 votes or less away, then Ted Cruz is going to be pretty far behind him. And maybe there's some other things that are going to be happening.

I would be fascinated to know as they get towards the finish line what sort of vice president are they going to pick? That could sway the direction one way or the other . I don't know, but that's why a place like CNN is going to have an awful lot of viewers over the next couple of months as this gets sorted out.

BLITZER: What do you think about Marco Rubio as a vice presidential pick potentially for Donald Trump?

CHAFFETZ: Hey, look, I supported him for the presidency. I support Marco for anything in between. I just think he's a wonderful, decent human being. He would have been a great president. So, he's always got my support, as far as I'm concerned.

BLITZER: You are in the House. But the Senate, it has got an issue it's got to deal with right now.

Senate Republicans, you think they are right or making a mistake for not even giving a hearing to the Supreme Court nominee, Judge Merrick Garland? You are familiar with his background, familiar with his record. How do you see it?

CHAFFETZ: Well, look, under advice and consent in the Constitution, there is nothing in the Constitution that compels people to have to have this hearing or to have a vote at a particular time.

And you have to look at precedent. And there's certainly -- the Democrats, they tried to have it two different ways. Look, we're in the midst of a political season, and I think the voters as they go into that ballot box in November will also be making a decision about the direction of the Supreme Court. That's the way the Senate is going to operate. The Senate gets to make that decision.

The powerful chairman of the Senate Judiciary Committee agreeing to meet with President Obama's Supreme Court nominee. Is it a crack in the Republican wall of opposition?

[18:15:07]

Plus, the GOP's Trump trouble. Will the party try to go around its own front-runner, and will it spark riots, as Trump is predicting?

(COMMERCIAL BREAK)

BLITZER: The breaking news this hour: top Republican Senate leaders speaking by phone with President Obama's U.S. Supreme Court nominee, Judge Merrick Garland.

Majority Leader Mitch McConnell telling him that he won't get a confirmation hearing, but there is talk of a possible meeting with Judiciary Committee Chairman Chuck Grassley.

Our justice correspondent, Pamela Brown, is working the story for us.

Pamela, what are you learning?

PAMELA BROWN, CNN JUSTICE CORRESPONDENT: Well, Wolf, we have learned the new nominee, Merrick Garland, began reaching out to senators shortly after the president made the announcement before he heads to the Hill tomorrow.

[18:20:01]

And now we're learning the Senate judiciary chairman, Chuck Grassley, is considering meet with Garland in person after recess. For the president, it is clear this decision is about his legacy, not the election.

(BEGIN VIDEOTAPE)

BARACK OBAMA, PRESIDENT OF THE UNITED STATES: Today, I am nominating Chief Judge Merrick Brian Garland to join the Supreme Court.

BROWN (voice-over): President Obama today made his case for 63-year- old Merrick Garland, chief judge for the D.C. Appeals Court.

OBAMA: I have selected a nominee who is widely recognized not only as one of America's sharpest legal minds, but someone who brings to his work a spirit of decency, modesty, integrity, even-handedness and excellence.

BROWN: An emotional Judge Garland with his family looking on introduced himself to the nation.

JUDGE MERRICK GARLAND, U.S. SUPREME COURT NOMINEE: This is the greatest honor of my life, other than Lynn agreeing to marry me 28 years ago.

BROWN: The White House touts Judge Garland as having more federal judicial experience than any nominee in history, serving nearly two decades on the bench. Before that, he prosecuted Timothy McVeigh in the Oklahoma City bombing case.

GARLAND: A Ryder truck was seen there.

BROWN: The president considers the Chicago native and Harvard Law graduate a consensus nominee. He was appointed to the D.C. Appeals Court by President Bill Clinton in 1997 and confirmed by a 76-23 Senate vote with bipartisan supporters, including Republican Orrin Hatch.

SEN. ORRIN HATCH (R), UTAH: Based solely on his qualifications, I support the nomination of Mr. Garland and I encourage my colleagues to do the same.

BROWN: The ball now is in the Republicans' court, and they vow Judge Garland will not get a hearing.

SEN. MITCH MCCONNELL (R-KY), MAJORITY LEADER: And it is the Senate's constitutional right to act as a check on a president and withhold its consent.

BROWN: For Garland, this day is especially poignant, having been passed over twice before for a seat on the high court.

GARLAND: Mr. President, it's a great privilege to be nominated by a fellow Chicagoan. I am grateful beyond words for the honor you have bestowed upon me.

(APPLAUSE)

(END VIDEOTAPE)

BROWN: And it's unusual for a president in recent history to nominate someone in their 60s. But this was considered Garland's last shot at the high court.

And, therefore, he has less to lose compared to other potential nominees in the running. And his age, Wolf, may possibly make him more palatable to Republicans because he won't be on the court as long as a younger nominee would be -- Wolf.

What's your take on the nomination? Is it really going to be stalled? Is it going anywhere at all?

JEFFREY TOOBIN, CNN SENIOR LEGAL ANALYST: Well, first of all, it is just worth pointing out that Merrick Garland is impeccably qualified to be a Supreme Court justice. If that were the standard, he really would not have any trouble getting confirmed.

However, the line of opposition to him in the Republican Party in the Senate seems implacable. It does not seem like there is any chance that this Senate will confirm him. The idea that he's getting some meetings maybe, that that's some sort of progress, seems almost sort of pitiful.

But the answer appears to be no. He does not appear to have any chance of getting confirmed.

BLITZER: Because they would need, Manu -- and you cover the Hill for us -- they would need a whole bunch of moderate Republicans to join the Democrats and get enough votes even for a vote on his confirmation.

Let's listen to Senator Rob Portman. He's a Republican from Ohio. Listen to what he said.

(BEGIN VIDEO CLIP)

SEN. ROB PORTMAN (R), OHIO: I meet with folks all the time and I'm not in the habit of turning down meetings. But if I do meet with him, it may not be a good use of his time, because I'm not going to change my position because it's based on the principle of this nominee having the possibility of reshaping the court for generations.

And I think it's best to allow the American people to weigh in on that.

(END VIDEO CLIP)

BLITZER: You see any Republicans at all? There's talk Susan Collins of Maine maybe, but any Republicans at all really joining the Democrats and saying this guy should have an up-and-down vote?

MANU RAJU, CNN SENIOR POLITICAL CORRESPONDENT: Not really, other than Mark Kirk of Illinois. He's in a blue state, Republican, of course in a very difficult Senate race. He said that he'd like to consider the nomination.

It's significant that someone like Rob Portman takes this hard line. He's in a very difficult Senate race this year, a blue state, somebody that's a risk of losing for certain. But they believe -- the Republicans who are up reelection by and large believe this is a good issue for them to run on.

[18:25:00]

Now, Democrats think they are out of step, but the Republicans really feel that if they can make the case to voters that they will be the firewall against a justice, it gives them an issue to run on. It will fire up their base. Kelly Ayotte, the New Hampshire Republican, I had a chance to speak to her too, also in a difficult Senate race, said the same thing.

Sure, she may meet with Merrick Garland, but she does not think this process should go forward. So, that's very significant. Mitch McConnell has been able to hold his troops in line as of now.

BLITZER: And Chuck Grassley, the chairman of the Judiciary Committee, he may meet with him, have a cup of coffee, but he's not going to have any hearings, certainly not going to allow any roll call votes.

So, Gloria, how is this battle over the president's pick for the Supreme Court going to play out, out there on the campaign trail?

GLORIA BORGER, CNN SENIOR POLITICAL ANALYST: Well, I think it's going to be a huge issue. I mean, you know, this nomination could reshape the direction of the court for at least a couple of decades.

And I think, unlike a lot of political discussions about, oh, the Supreme Court is important, this is real. This is tangible. This is the replacement for Justice Scalia. This is about the balance of power.

So the stakes are obvious. And I think, you know, the Republicans believe that this will motivate their base, and they are probably right about it. But the Democrats also have a case to make that this will motivate their base. I think, look, the Democrats can make the case that the president compromised to a degree on tenure, because Garland is 63 years old, on ideology, because he's not as liberal as some on the left in the party would like.

And they effectively dared the Republicans to come out and oppose him, because so many of them have supported him in the past. So the Democrats can make the argument to their base that this is about fairness, about getting a fair hearing, about even getting a meeting or a cup of coffee and that this is about the Constitution.

And so I think it will be a motivating factor on both sides.

BLITZER: The fact, David Chalian, that the Republican majority leader in the Senate, Mitch McConnell, won't even have a meeting with him, give him the courtesy of at least coming over to his Senate office and having a conversation, how is that going to play out politically?

DAVID CHALIAN, CNN POLITICAL DIRECTOR: Well, that's kind of posture, to what Manu was saying, is how he's keeping his troops in line, just absolute line in the sand. No movement on it. I'm here to give all the Republican senators cover on this is McConnell's point.

But to Gloria's point, I just want to pick up on what she said. It's a different thing. It's a base play move on the Republican side, but because the pick is who he is and not some liberal darling, it's not a base play move that the president offered Hillary Clinton or Bernie Sanders to run with.

They're trying to gin up excitement in their base as well. Notice Hillary Clinton's statement today. She fully supports the nomination, credits his qualifications, but her entire statement was about the process, just trying to get her troops excited to demand that the Republicans give him a hearing. That's very different than sort of championing and sort of taking this nominee on the trail with her and trying to build up enthusiasm that way.

This pick doesn't seem to be able to do that. If I read MoveOn's statements and other liberal groups, they are much more focused on the process than they are on the pick.

BLITZER: All right, guys, stand by. We're going to get more on this.

Also, we're getting some more information on Donald Trump and his path to the Republican presidential nomination. Stay with us.

(COMMERCIAL BREAK)

BLITZER: Donald Trump deciding today not to take part in the next Republican presidential debate, claiming he only recently found out about it. His rival, John Kasich, saying he won't take part without Trump being there, leading FOX News to cancel the contest -- the debate, I should say.

[18:33:14] David Chalian, we've done some estimates. I know you and your team have been looking at this closely. Trump needs, what, 57 percent of the remaining delegates out there to clinch the nomination going into the convention in July. But that number almost certainly will drop. Explain.

CHALIAN: We aren't done allocating all of the delegates from the contest last night. So, obviously, what our team does is they look to make sure every vote is counted, and then you have to look at it congressional district by congressional district to follow the delegate allocations.

So we've got, once those last delegates are allocated, we anticipate, just looking at the trends, that that number, his target number will probably go down. It will probably be somewhere around 53 percent of the remaining delegates or 54 percent that he will need to get to 1,237 which, by the way, is doable. It is not impossible. It is a steep climb, but it is doable before we get...

BLITZER: In contrast, Cruz and Kasich, they need a much, much higher percent.

CHALIAN: It's almost not possible.

BLITZER: Yes. That's what most of the analysts have suggested.

Gloria, Kasich says there's now a three -- it's now a three-person race. He's the only candidate, he says, that actually beat Hillary Clinton in the big states in a general election like Ohio, Pennsylvania, New Hampshire, other states. Does Kasich really realistically, though, have a path forward?

BORGER: Well, you were talking about the steep climb with David. I mean, Kasich would have to win over 100 percent of the delegates in order to get to the magic number. So he has -- he has a very steep climb.

Look, you have these two candidates, Cruz and Kasich, who have very different appeals. And if they were to somehow get together to try and stop Donald Trump, Kasich has appeal in the mid-Atlantic states, the kind of bluer states. Cruz has some appeal in the red states, the western states coming up. And so if both of them stay in the race, as lots of people are suggesting they do, they might be able to tamp down Trump's final number heading into the convention.

[18:35:22] The question is, Wolf, what happens next? What happens if they then do that and they keep Trump a few hundred delegates away? Then what? Then what happens at a contested convention? Do you deny the nomination to Donald Trump? I think that's a very difficult case to make at this point.

BLITZER: Well, speaking of that, Jeffrey, earlier today on "NEW DAY," Trump actually warned there could be riots. Riots. He used the word twice, if the party were to block him at the convention. What was your reaction to that?

TOOBIN: Well, my reaction is, we have been outraged at Donald Trump's outrageous statements about John McCain, about Megyn Kelly. Over and over again. And none of it seems to make any difference.

Here's a candidate whose rallies have been full of violence, and here he is predicting riots if he is not nominated. I mean, it's completely outrageous, but based on everything that's happened so far it will have absolutely no impact on his support in races -- in the primary races going forward.

BLITZER: Manu, based on everything you're hear, is a contested convention almost certainly going to happen, or do you think Donald Trump will have that 1,237, the magic number needed to get the nomination?

RAJU: I think it's possible he could get that 1,237, particularly if Kasich and Cruz split sort of that anti-Trump vote. But I think the contested convention scenario actually scares a lot of Republicans here in Washington. I spoke to a lot of Republican senators about this. And what they're really worried about is a real nasty fight on the floor.

How does it end up? Pretty unpredictable. And then alienating millions of Trump supporters if Trump does not end up the victor. So that's the scenario that is certainly floated, but it's only one and not necessarily preferred by Republican leadership, even if they're worried about Donald Trump as the nominee.

CHALIAN: Wolf, remember what a convention is and when it falls. Yes, it is to nominate the candidate, but it is also the launching pad to the general election. That is when you have the best opportunity to get before the American people and start telling the story of your nominee and your party.

And if the story the Republican Party is telling is a total chaos and a fight on the convention floor, they're going to be so behind the eight-ball to start the general election. That concerns a lot of Republicans I speak to.

TOOBIN: But they are also concerned about Donald Trump as the nominee.

CHALIAN: Obviously. Right.

TOOBIN: That's -- that's the problem. I mean, you have "The National Review," which is sort of the bible of conservative thought, put out an entire issue, nothing but attacks on Donald Trump. You know, you have Marco Rubio, who you know, called him things that are outside the usual bounds of politics: liar, con man. And this is not an ordinary race.

BORGER: You know, this is their nightmare, and they're living through it. And, you know, the Republican Party is collectively pulling its hair out about Donald Trump. They were late. If they disliked him so much, they needed to get involved in this a little bit earlier.

I think the question is, weren't they a little late to this game? They were all presuming, as were many of us, quite frankly, that Donald Trump would implode. He didn't. He succeeded. He's touched a nerve. He's brought new voters into the Republican Party.

And at some point, they are going to have to decide that they need to live with the candidate that their -- that their base voters are supporting.

BLITZER: Rubio is going to endorse -- is certainly not going to endorse Donald Trump. You think that Rubio might endorse Kasich or Cruz, for that matter?

RAJU: Well, it's hard to know right now. Rubio actually would be asked a lot about Kasich on the campaign trail, and he would not criticize Kasich. He was very careful in doing so. Of course, he criticized Ted Cruz pretty relentlessly. But we'll see how he ends up playing forward -- going forward.

But it's interesting. When we talk about the scenarios for the party, all three scenarios that we're looking at are pretty disastrous, according to Republican leadership and Republicans here in Washington. You have Donald Trump who says inflammatory things and could scare their down-ticket races. Ted Cruz, who's very conservative and may not play well in purple and blue states. And then if Kasich becomes the nominee, you have to go through that messy convention scenario. So nothing really looks good right now.

[18:44:37] BLITZER: The two Democratic presidential candidates just wrapped up another big delegate battle. And Hillary Clinton is confident that her campaign is on track right now to win the nomination.

But Bernie Sanders is vowing to stay in the race, citing a favorable schedule during the coming weeks.

Our senior Washington correspondent, Joe Johns is following the Democratic campaign. He's joining us from Phoenix, Arizona, right now. What's the latest, Joe?

[18:45:01] JOE JOHNS, CNN SENIOR WASHINGTON CORRESPONDENT: Well, Wolf, the Bernie Sanders campaign was using sports analogies today. They say this is a big game, and at the end of the game, they are going to win. And they base that on the fact that the electoral map will expand especially out to states like Arizona, which they see is in their favor. They say this is still a race for delegates, even as Hillary Clinton was enjoying enormous victories in multiple states.

JOHNS (voice-over): Hillary Clinton sweeping Super Tuesday scoring four victories in four states with Missouri still too close to call.

CLINTON: We are moving closer to securing the Democratic Party nomination and winning this election in November.

JOHNS: Clinton's big night gives her a clearer path to the Democratic nomination and blunts Bernie Sanders' momentum coming out of his Michigan primary victory last week.

Clinton holds a practically insurmountable lead when it comes to the delegate count, nearly doubling Sanders. She is just shy of the number she needs to clinch the nomination.

SEN. BERNIE SANDERS (I-VT), DEMOCRATIC PRESIDENTIAL CANDIDATE: Next week, Arizona has a very important election.

JOHNS: Despite his delegate deficit, Sanders is looking forward to future contests and not letting up on his rival.

SANDERS: My opponent raises money in a slightly different way. She has a super PAC which, among other special interests, has received $15 million from Wall Street.

JOHNS: But Clinton seems ready to move on to the general election, now focusing squarely on the current Republican front-runner Donald Trump. CLINTON: When we hear a candidate for president call for rounding up

12 million immigrants, banning all Muslims from entering the United States, when he embraces torture, that doesn't make him strong. It makes him wrong.

(CHEERS)

JOHNS: And Trump appears eager for the fight, firing back at Clinton this morning.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE (via telephone): I think she's an embarrassment to our country. She's under federal investigation. She doesn't have the strength or the stamina to be president, frankly, as far as I'm concerned.

JOHNS: The war of words offering a preview of a potential November showdown between the two. Trump also targeting Clinton on social media today, posting a video saying Clinton isn't a leader. She's a punch line.

(BARKING)

(END VIDEOTAPE)

JOHNS: The Sanders campaign is expected to push hard here in the state of Arizona. Campaign officials say don't expect him to change his tone, vis-a-vis Hillary Clinton. The Clinton campaign, meanwhile, rolled out three new ads targeting the state -- Wolf.

BLITZER: Yes, Arizona, the big contest there next Tuesday. We'll be watching that closely. All right. Joe, thanks very much.

Let's bring back our experts.

Jeffrey, what's was your reaction to that latest Trump Instagram video making fun, shall we say, of Hillary Clinton?

TOOBIN: I thought it was totally moronic. I thought it was just so silly as to be ridiculous, and also, not persuasive to anyone who doesn't hate Hillary Clinton already. You know, she was barking as part of an imitation of something from Arkansas earlier. I mean, the idea that someone's vote is going to change based on that just seems absurd to me.

BLITZER: Well, Gloria, as you know, Hillary Clinton is now spending increasingly more time going after Trump. Not necessarily going after Bernie Sanders right now. What does that say to you?

BORGER: I think she is, as we say, pivoting to the general election. Look, she believes that Donald Trump can coalesce her support like nothing else. They, in the Clinton campaign, they obviously don't want to be cocky about running against Donald Trump, and I don't think they should be because I think he would be a tough competitor for Hillary Clinton. But nothing so unites Democrats as talking about Donald Trump.

So, why wouldn't she do it? It makes a lot of sense.

BLITZER: You know, the problem, though, from -- you attack Donald Trump, David Chalian. As you well know and other Republican candidates have done it. He hits you right back, and he goes really after, you know, some very sensitive points.

So, more Hillary Clinton goes after Trump, the more he's going to go after her. This could get really, really ugly.

CHALIAN: That's is true though those attacks will be different in the context of a general election than it will be in the context of a Republican primary because, of course, her attacks will get her base excited and probably come to her defense a lot when he comes after her.

Listen, that Instagram video is a ten-second video. That's to rally his troops. That's not a television ad where he's trying to persuade independent voters to come to his side. He's just trying to rally his troops with an Instagram video.

Defense a lot when he comes after her. Listen, she already has been on the receiving end of an attack from Donald Trump. Last December he came after bill Clinton and came after her.

BLITZER: When he said he was sexist.

CHALIAN: And then they got really quiet about that. So, she -- it was her first moment of understanding what it was going to be to box with Donald Trump. And she probably -- and her campaign probably has a lot more to learn about that by watching the Republican --

RAJU: And one thing good for the Clinton campaign is they struggled with enthusiasm particularly among younger progressives, Donald Trump will bring Democratic voters out to the polls. It's clearly something that they're banking on if there is a general election matchup between the two.

BORGER: Yes, I think it does. And I think, you know, you look at the sum total of the ads Trump has put out or that the anti-Trump PACs have put out against him or the Democrats have put out. This is going to be a knock down drag out fight, to use the cliche.

And that, you know, she understands to David's point, that he's going to pull out every stop, and she may have to as well. Because if he's going to start talking about Bill Clinton and Monica Lewinsky, Hillary Clinton is going to be talking about Trump University and everything else. And, you know, I think we've only just seen a little bit of what could occur in that general election matchup.

CHALIAN: And these 24 candidate are two candidates with high unfavorables already.

BORGER: Exactly.

CHALIAN: So, imagine what it will be like in the end of the cycle.

BORGER: Right.

BLITZER: And, remember, you can imagine -- you can imagine those three scheduled presidential debates in a general election in September and October, if it's Hillary Clinton and Donald Trump going, you know, face to face in a presidential debate?

TOOBIN: Won't we enjoy that? That would certainly be highly rated debates, but also, the business of attacking Donald Trump is actually turning out to be quite difficult. Marco Rubio experimented with matching him for vulgarity for vulgarity and it only hurt him, so Hillary Clinton is going to have to figure out what works best for her. And it's no obvious.

BLITZER: All right, guys. Stand by, we're going to continue to follow obviously this race for the White House.

Also coming up, a terror raid in France leads to four arrests. Were the suspects plotting a sequel to the horrific massacre in Paris? We're getting new details. Stand by.

(COMMERCIAL BREAK)

[18:57:24] BLITZER: Europe is on edge tonight following a pair of terror raids, including a shootout in Belgium that left one man dead and two suspects on the loose.

JIM SCIUTTO, CNN CHIEF NATIONAL SECURITY CORRESPONDENT: Well, there were four people arrested today in Paris. They were suspected of planning a terror attack in the capital. The group discussed launching this attack and making contact with ISIS. The weapons they found, just a starter pistol, one bullet. No other weapons.

But here's what's truly remarkable. One of the four was actually under house arrest as he was planning this attack. He had been convicted in 2014 for planning to go to Syria to join the al Qaeda affiliate Jabhat al-Nusra and this has real questions in France about the state of emergency that they were watching this guy and yet he was able to at least get into the early stages of planning an attack.

BLITZER: Shocking. So, what's the threat level in France like right now?

SCIUTTO: It's extremely high now. French officials describe it as unprecedented. I met with the French interior minister last week. He said another successful terror attack in France in 2016 is likely. He told us, however, that France has foiled some 12 plots just since the start of 2015. Six of those plots in the last spring and summer alone, that period, of course, between the "Charlie Hebdo" attacks in Paris and then the November attacks when so many were killed there as well.

BLITZER: This comes, what, a day after a police raided an apartment where in Brussels that had some connection to the Paris attacks?

SCIUTTO: That's right. This was in Brussels. It was in Belgium next door to France. But French police were involved. They raided an apartment connected to the November attacks in Paris.

Once they arrive in that apartment, people inside that apartment opened fire on them, killing one person. The police killed one. The raid turned up an ISIS flag, a Kalashnikov rifle, ammunitions as well. This is what's alarming, Wolf. That two people did get away. They're still on the run, and as we know, Belgium is a real hot point for terror groups.

BLITZER: NATO headquarters happens to be there, which raises concerns here in the United States. They see what's going on, law enforcement here, counterterrorism experts in Belgium and in France. They worry that it could happen here in the United States.

SCIUTTO: No question. They certainly worry about it and the U.S. is giving them enormous help to help them stop the next attack.

BLITZER: We've heard from the director of national intelligence and others that clearly worried about a potential attack here this year.

SCIUTTO: No question the U.S. as well.

BLITZER: All right. Jim Sciutto, thanks very much for that.

That's it for me. Remember you can always follow us on Twitter. Please tweet me @wolfblitzer. You can always tweet the show @CNNSitroom.

Please be sure to join us right here in THE SITUATION ROOM tomorrow. Thanks very much for watching. I'm Wolf Blitzer.